Insensitive comments...

[rube the noob]

I got my hammerhead working but it isn't very sensitive. It will pick up a big hunk of metal at about 6 inches (ie a bench vice). A small piece will get a couple clicks at point-blank range. So I have come to solicit advice as to what my next move should be. My suspects are:

Coil Shielding. I started with foil, the kind you use for a radiant barrier in an attic. It didn't detect much. After fooling around with the settings, I took it off and tried it without any shielding. It seemed to work better. Then I tried a non-static bag, which didn't make too much difference. I wouldn't think inadequate shielding be the cause of this type of problem, but could it?

My connections from coil to unit. I am using an old TV coax cable with a connector I took off an old pc card. (f-type I guess its called) I had hoped this would make it easier to change coils in the future, but it might be making a sloppy connection. Could I be introducing a problem by this setup?

The electronics. Could be any sort of novice mistake. It seems to pass all the initial tests Carl lists in the instructions. I thought I saw some pictures of various scope traces posted on the board, but I can't find them now. This is the least desireable to dig into first, since I don't want to goof anything up that is working, but I don't want to waste time. If this is a likely culprit, I will dig into it.

Adjustments. I put most of the pots on the unit, so I can easily access them. Could my problems be simply that I don't have it "tuned up"? I fooled with everything quite a bit, but may have missed the sweet spot. I have never used a metal detector before, so I don't know how finicky they are.

Obviously I am not looking for someone to tell me my exact problem from such vague description, but hopefully someone could point me in the direction of greatest probability of finding the problem. Should I start in any of the above directions, or should I go back and reread the old threads?

[Reg]

Hi Rube,

There could be several reasons why your detector is "insensitive". For starters, you might want to try to see what happens when you remove the shielding and do a simple air test. If things change dramatically, then the shielding may be the problem. You can't have a shield that is completely shielding the coil. In other words, if you wrap foil around the windings, then there has to be a gap in the foil around the circumference. This gap only has to be a small amount, maybe a quarter of an inch or so, but it has to be there. If there is no gap then the shield acts as a shorted turn and screws things up.

Any use of aluminum for shielding has to be wrapped around the windings. If it is just placed under the coil and spaced a ways from the winding, then the foil acts like a target and will cause the signal to become saturated. This will show up at TP 5.

My guess is the problem is really a series of things. For starters, what is the value of your damping resistor and what is the inductance of your coil? These two components will determine your minimum sampling delay, which has a major impact on the sensitivity.

Do you have a scope and if so, what does the waveform look like at the coil and at the output of the preamp? Those two signals will tell a lot. It could be you have some oscillation and that could cause what you are experiencing.

Next, you need to adjust a few things for maximum sensitivity. For starters, you can simply put the pulse length at mid point or less. Next, turn the delay towards minimum until the unit doesn't work or barely works and then extend it back out until the signal becomes suddenly stronger. This minimum delay setting is critical. If you happen to sample too soon, then the detector will appear to be "dead" and have little or no sensitivity, much like you are experiencing.

Make your subtract signal, the second sample much later. If it is too close to the main sample, then it can reduce your sensitivity. R 35 has a dramatic impact also.

Check the voltage at the output of IC 8, which is TP 5. This voltage has to be less than 4 volts. It is preferable that this voltage be near 0V. If this voltage is over 4 volts, then it is possible the amp is in saturation. If it is, then the sensitivity will dramatically lower, much like you are experiencing. If your shielding is being detected, TP 5 will show a voltage of over 4 volts, maybe 4.2 or 4.3V or so. If this is happening, then the amp is in saturation and the detector will act "dead" or have no sensitiivty at all. It could be saturated because the detector is detecting the shielding or because the first sample is taken too soon. If there is oscillation at that preamp, then this could cause the same results, and depends upon when the sample is taken.

If you have a scope and a digital camera, then you can take a couple of pics of the pulse and the output of the preamp and post them. That will help dramatically.

If you do not have a campera but do have a scope, you might want to compare your signals to those shown in the HH article and let us know any differences.

Hope this helps.

Reg

[rube the noob]

Thanks Reg,

Here is a pic of the voltage across the coil and the output of the preamp. Something looks funny about the preamp output. It shouldn't have that little kick up at the end of the wave, should it?

[Reg]

Hi Rube,

I am not sure what I am seeing on the pics. Is the signal from the coil going to pin 2 of the preamp? If so, then the pic appears to be upside down. Do you have a switch called invert on your scope? The signal from the coil looks inverted. To make matters worse, the second pic almost looks like there is no amplification at all at the preamp.

Check so see if you have the invert on and if so, turn it off and then take and post the pics again.

Reg

[Carl-NC]

Yeah, the opamp output looks way wrong. Pull the opamp out, check the waveform at TP3. If it looks like the waveform at TP1, then your diodes are blown, and that means your opamp is probably blown, too.

- Carl

[rube the noob]

Sorry, I should have labeled better. The first picture is the voltage across the coil at the point of the connector and coax (off the board). looking back, I put the ground for the scope probe on the core of the coax, so I believe the upside down wave is just due to the probe being hooked up backwards.

I'll go check the other stuff....


Thanks guys!

[rube the noob]

OK, here are a few more pics...

These are TP 3 and TP 4. I didn't adjust any settings on the scope, just moved the probe. (I did adjust settings on the previous set of pictures) It looks like the opamp is inverting and amplifying, so it must be a problem with prior circuitry, correct?

[rube the noob]

In this set I pulled the opamp and checked TP3 vs TP1, which are posted below. They are different, but the output of TP1 looks fishy. Isn't it kind of sloppy for a clock output?

[Carl-NC]

Yes, TP1 looks odd. Are you using a CMOS 555 or a bipolar version?

On second thought, the opamp output might be close, but it does appear slightly underdamped. Might need to reduce R11.

On third thought, you might have a misadjusted oscope probe. That would explain the exponential responses on all the waveforms. Do you have a ~1kHz square wave generator you can probe?

- Carl

[rube the noob]

Carl, yer a genius. It was my scope probe. It was on 10x, so after putting it on 1x I got results that look like the ones in the instruction guide.

TP 4 I still have a question about. When I lengthen the wave, it looks like Carl's picture, but on this setting there is that crazy discontinuity. The funny thing is it looks like it alternates a bit in voltage every other wave. Is this normal?